Abstract: The most direct reflection of the underground medium density change is the surface gravity change, in this article water flooding data from 1995 to 2007 of every oilfield of Zhongyuan oilfield in Puyang area is collected and studied.The change of gravitational field caused by water flooding is calculated, and the relationship between it and seismicity is studied.The result indicates that the gravitational field change caused by water flooding is about ±1~10×10^-8 m·s^-2 and gradually attenuates with the distance from the center of oil field.